Older Women More Likely to Have Twins

Feb. 22, 2006 -- As women age, their pregnancies are more likely to yield twins -- but why?

Fertility drugs and family history of twins are known to boost a woman's chances of having twins. Hormonal changes that come with age are another reason, a new study states.

The study comes from researchers at Vrije University Medical Centre in the Netherlands. They included professor Roy Homburg of the university's division of reproductive medicine.

Homburg and colleagues found that older women have higher levels of a hormone called FSH, prompting greater likelihood of having fraternal twins. The study appears online in Human Reproduction.

Hormone Shift

Data came from 507 women aged 24-41 who were undergoing intrauterine insemination due to unexplained infertility or mild male infertility. The researchers measured the women's hormones and checked ultrasound images of the women's reproductive system during a total of 959 cycles of ovulation.

Homburg's team compared women in their 20s with women in their 30s. Their findings:

• Older women had higher levels of a hormone called FSH (follicle-stimulating hormone).
• Older women were more likely to prepare more than one egg per menstrual cycle.

Those patterns were strongest in women aged 35 and older, followed by women aged 30-35, and lastly by women in their 20s.

Fraternal twins develop when two eggs are fertilized. So if older women are more likely to produce two eggs per cycle, they're also more likely to have fraternal twins, the researchers argue.

The study doesn't cover identical twins, which are much rarer and develop from one fertilized egg.

Birds, Bees, Hormones, and Age

Want a little more background?

During the follicular phase of a woman's cycle (which starts on the first day of her menstrual period), FSH and another hormone, called luteinizing hormone (LH), are released from the brain and travel through the blood to the ovaries. They stimulate about 15-20 eggs in the ovaries into individual "shells," called follicles.

Usually, only one of those follicles matures and is released during a woman's cycle. Think of follicles as a field of fledgling candidates, with only one getting the nod to go forward. Sometimes, more than one follicle gets released.

SOURCES: Beemsterboer, S. Human Reproduction, Feb. 23, 2006; online edition. WebMD Medical Reference in collaboration with The Cleveland Clinic: "Sexual Health: Your Guide to the Female Reproductive System." News release, MW Communications.